In certain situations and circumstances it is important to know if the insulation surrounding a wire or cable has deteriorated. Such deterioration could be a precursor to a failure in an important system. For example, in jet fighter aircraft, wire chaffing and the resulting deterioration of the insulation are often precursors to failure. If the chaffing continues undetected, the cable may be severed or shorted to another cable with catastrophic results depending on the purpose of the cable. If the cable is used to control the rudder or aileron, severing or shorting that cable could result in a loss of control of the aircraft and possibly a fatal crash. It would be desirable therefore to have a method and device for detecting the deterioration and failure of wire insulation. If insulation defects and deterioration are detected in a timely manner, the wire or cable could be replaced before a catastrophic failure occurs.
U.S. Pat. No. 3,096,478 discloses an apparatus for detecting non-uniformity in electrically insulated wires through the use of conductive gas electrodes. The electrodes consist of a tube or sleeve containing ionized air which establishes a direct current path through the defective insulation segment. In the apparatus disclosed in this patent, the cable must be placed inside the conductive gas electrode. Similarly, the cable must be placed inside the nonconducting tube containing a semi-conducting ionized gas to test the insulation in the apparatus described in U.S. Pat. No. 3,263,165. The methods and devices of evaluating wire insulation disclosed in these two references require the measurement of very small currents in the conductor of the cable under test. They require that the equipment normally connected to the cables under evaluation be inoperative and that the cables be disconnected from the equipment in most cases. Such requirements considerably limit the number of applications where such cable insulation evaluation methods and devices can be used.
U.S. Pat. No. 3,639,831 discloses a method and apparatus for producing a directable, electrically conducting gas jet and detecting the presence of anomalies therein caused by insulators, conductors or semi-conductors. The gas jet flows across a test zone and impinges upon a target anode which is maintained at a bias potential with respect to the cathode of the ionizing generator such as to cause an electrical current to flow between the anode and the cathode via the gas jet across the test zone. This device requires that the electrical current flow between the gas jet nozzle and the target anode be constant. Moreover, there is no provision for measuring anomalies which have their own potential or are carrying a current.
It would be desirable therefore if an insulation integrity evaluation could be performed while the equipment connected to the cables under test were not disconnected or were operating and the normal potential waveforms were present on the conductors.